As described in U.S. Pat. No. 8,348,495, “a metallic glass alloy is an alloy that includes elements satisfying specific conditions and having a metallic element as a main component, and is an amorphous metal alloy with a disordered atomic-scale structure. Such metallic glass alloys are formed, for example, by cooling the molten raw materials at a critical cooling rate of 104 K/s or greater. The properties of these metallic glass alloys include high wear resistance, high strength, a low Young's modulus, and high corrosion resistance.”
Thermoplastic forming (TPF) based processing has been already suggested in the early days of metallic glass research as a method for forming1 and has been widely used ever since2. It is based on the existence of a supercooled liquid region, the temperature region above the glass transition temperature where the metallic glass former exists as a (supercooled) liquid before it eventually crystallizes during further heating. This supercooled liquid region (SCLR) in metallic glass formers, and thereby TPF, is unique among metals.
The maximum strain that can be achieved during TPF is called the formability (for given conditions, stress, geometry) and is limited by the metastable characteristic of the metallic glass (or when relaxed, the supercooled liquid region3). The formability of a metallic glass in its SCLR can be described by the maximum strain the MG can undergo before it eventually crystallizes. Under the assumption of Newtonian behavior, i.e.,σ=η3{dot over (ε)}  (1)where σ is the flow stress and {dot over (ε)} is the strain rate and under isothermal conditions the maximum strain can be calculated by integrating Eq. (1) between 0 and tcryst:
                                          ɛ            .                    ⁢                                          ⁢          d          ⁢                                          ⁢          t                =                              ∫            0            cryst                    ⁢                                    σ                              3                ⁢                                                                  ⁢                η                                      ⁢                                                  ⁢            d            ⁢                                                  ⁢            t                                              (        2        )            
Thus the maximum strain achievable under isothermal conditions is given by:
                              ɛ          max                =                              t            cryst                    ⁢                      σ                          3              ⁢                                                          ⁢              η                                                          (        3        )            
Here, the isothermal formability is given by:
                    F        =                              t            cryst                                3            ⁢                                                  ⁢            η                                              (        4        )            
DE102011001783 describes an amorphous strip material which is used as an elevator spring and which initially prepared with a melt spinning process, preferably as a continuous tape or film in a thickness of typically 50-200 μm. This amorphous strip material has a high strength and a low elastic modulus and can be made under normal atmospheric conditions; heat treatment under vacuum or inert gas is not required.
DE102011001784 describes an amorphous alloy which can be used, e.g. as an elevator spring and which preferably has a crystallization temperature Tx of greater than 400° C., an amorphous ribbon material which is first produced with a melt spinning process as a continuous strip or foil with a thickness of, for example, about 40 to 200 μm. The amorphous alloy can be directly cast as amorphous ribbons by treatment steps providing a better and more uniform surface structure, in particular with a reduced surface roughness, and a smaller number of surface defects and defects, as well as a uniform, typically rectangular cross-section. In one process described in DE102011001784, shaping is performed by heat treatment, preferably at a temperature of between 0.3 to 0.7 Tx. This temperature range provides a sufficient diffusion of the required for the shaping relaxation, which is required for the embossing of a mainspring form. In this temperature range, there is no crystallization of the amorphous material, which would be accompanied by undesirable brittleness of the strip material. The duration of the heat treatment, depending on temperature, can be from one minute to four hours.
U.S. Pat. No. 8,348,496 describes a mainspring for a mechanism driven by a motor spring, especially for a timepiece, wherein the mainspring is a single monolithic metallic glass ribbon having a thickness greater than 50 μm, wherein the monolithic metallic glass ribbon has a spiral-shaped curvature in a free state of the mainspring. The ribbons intended to form the mainsprings are produced by using the quench wheel technique (also called planar flow casting), which is a technique for producing metal ribbons by rapid cooling. A jet of molten metal is propelled onto a rapidly rotating cold wheel. The speed of the wheel, the width of the injection slot and the injection pressure are parameters that define the width and thickness of the ribbon produced. Other ribbon production techniques may also be used, such as for example twin-roll casting. In the example described in U.S. Pat. No. 8,348,496, the alloy Ni53 Nb20 Zr8 Ti10 Co6 Cu3 is used. 10 to 20 g of alloy are placed in a delivery nozzle heated to between 1050 and 1150° C. The width of the nozzle slot is between 0.2 and 0.8 mm. The distance between the nozzle and the wheel is between 0.1 and 0.3 mm. The wheel onto which molten alloy is deposited is a wheel made of a copper alloy and is driven with a tangential velocity ranging from 5 to 20 m/s. The pressure exerted to expel the molten alloy through the nozzle is between 10 and 50 kPa. The ribbons are subsequently formed into their final dimensions by grinding or wire electrical discharge machining (WEDM). Finished ribbons are formed by a fitting operation whereby the ribbon is heterogeneously deformed into the final shape and heated at a temperature T where Tg−50<T<Tx+50.
While known TPF processes such as those illustrated by DE102011001783, DE102011001784 and U.S. Pat. No. 8,348,496 provide a variety of ways to cast amorphous alloys, the need continues to exist for processes that deform amorphous alloy ribbons under conditions (temperature and strain rate) that result in homogenous deformation, thereby minimizing processing defects and enabling the manufacture of a variety of conventional or customized articles.